Evaluating the potential of photo-identification as a monitoring tool for flapper skate (<i>Dipturus intermedius</i>)

Abstract Flapper skates (Dipturus intermedius) were once widespread in European shelf waters but are currently classified as critically endangered by the International Union for Conservation of Nature due to historical overexploitation. Novel monitoring approaches are needed to assess the efficacy of management measures, such as dedicated marine protected areas, for the conservation of relict skate populations. Flapper skates possess distinctive dorsal spot patterns, which could potentially be used for individual recognition using photo-identification (photo-ID) approaches. This study assessed the potential of photo-ID as a method for individual recognition of a relict population of skates within a dedicated marine protected area in western Scotland (UK), which has long been targeted by directed recreational angling. A collection of 486 photographs of 373 separate skate capture events from 2011 to 2016, taken with standard mobile phones and compact cameras, was studied using visual pairwise comparison methods to determine number of individuals and recapture rates. Results indicated that adult flapper skates were individually recognizable with a high degree of certainty through comparison of spot patterns, assuming appropriate lighting conditions. A total of 226 individuals were identified, of which 77 (34%) were recaptured at least once. The average recapture interval was 308 days (SE: 29.4 days), with the longest recapture interval to date being 4.4 years. Spot patterns among recaptured tagged or otherwise uniquely identifiable skates were found to remain stable over timescales of months to >1 year. Results indicate that photo-ID, based on photographs sourced through citizen science approaches, can provide a low-cost alternative means of monitoring flapper skate presence and distribution for the purposes of underpinning management decisions.